The disclosure of Japanese Patent Application No. 2001-175401 filed on Jun. 11, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
This invention generally relates to a brake control system of a vehicle such as an automobile. More particularly, the invention relates to a brake control system that ensures driving stability of a vehicle when braking the vehicle while turning.
2. Description of Related Art
Conventionally a brake control system of a vehicle, such as an automobile, which controls front and rear wheel braking force distribution, such that the yaw rate of the vehicle, becomes a constant target yaw rate when braking the vehicle while turning. This technology is disclosed in Japanese Patent Application Laid-Open Publication No. 6-127354. This brake control system is intended to improve the driving stability when braking the vehicle while turning by preventing the yaw rate of the vehicle from increasing at that time.
According to the brake control system disclosed in JP 6-127354, however, complex calculations are required which makes it difficult to apply to actual vehicles. Further, this brake control system controls the front and rear wheel braking force distribution such that the yaw rate of the vehicle becomes a constant target yaw rate when braking the vehicle while turning by reducing the braking force of the rear wheels. As a result, the deceleration of the vehicle is reduced compared to when front and rear wheel braking force distribution control is not performed.
An object of the invention is to easily and effectively prevent an increase in a yaw rate of the vehicle when braking the vehicle while turning without deteriorating the deceleration of the vehicle by controlling the front and rear wheel braking force distribution with a simple calculation without relying only on the control for decreasing the rear wheel braking force.
According to one aspect of the invention, a method and brake control system for a vehicle includes a first detector that detects a lateral acceleration of the vehicle, a second detector that detects a speed of the vehicle, and a controller that controls a braking force applied to each of front wheels and rear wheels. The controller increases the braking force applied to at least the front wheels so that the ratio of the braking force applied to the front wheels versus the braking force applied to the rear wheels increases as a product of the detected lateral acceleration and the detected speed of the vehicle increases.
According to the aspect of the invention, the controller increases the braking force applied to the front wheels when an absolute value of a product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than a first predetermined value and is less than a second predetermined value. The controller increases the braking force applied to the front wheels and decreases the braking force applied to the rear wheels when the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than the second predetermined value.
According to the aspect of the invention, the controller corrects an amount of increase in the braking force applied to the front wheels on the basis of the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle so as to prevent the braking force applied to the front wheels from exceeding a predetermined range when a braking amount of the vehicle by an operator of the vehicle becomes large.
According to the aspect of the invention, the controller determines whether the vehicle is turning while braking, and controls the braking force applied to at least one of the front wheels and rear wheels on the basis of the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle when it is determined that the vehicle is turning while braking.
In the aforementioned aspect of the invention, the yaw rate Yr of the vehicle can be expressed by Equation 1 below:
Yr=(1+Khxc3x97Vxc3x97V)|xc3x97(xcex8xc3x97V)/(Nxc3x97L)xe2x80x83xe2x80x83(1)
where Kh represents a stability factor, V represents a vehicle speed, xcex8 represents a steering angle, N represents a steering gear ratio, and L represents a wheel base.
Multiplying both sides of Equation 1 by (1+Khxc3x97Vxc3x97V) results in Equation 2 below. Supposing that the lateral acceleration of the vehicle is denoted by Gy, Equation 3, as shown below, can be obtained using the relationship Gy=Yrxc3x97V.
Yr+Khxc3x97(Yrxc3x97V)xc3x97V=(xcex8xc3x97V)/(Nxc3x97L)xe2x80x83xe2x80x83(2)
Yr=(xcex8xc3x97V)/(Nxc3x97L)xe2x88x92Khxc3x97Gyxc3x97Vxe2x80x83xe2x80x83(3)
Generally, when braking is performed while steadily turning in an actual vehicle, the vehicle tends to pull inward toward the inside of the turn regardless of whether the steering angle is constant. This phenomenon, that is, the increase in the yaw rate Yr is undesirable.
When applying this phenomenon to Equation 3 above, supposing that the change in the vehicle speed before and after braking operation is small, the yaw rate before braking denoted as Yr1 and the yaw rate after braking denoted as Yr2 can be expressed by Equations 4 and 5, respectively, below.
Yr1=(xcex8xc3x97V)/(Nxc3x97L)xe2x88x92Kh1xc3x97Gyxc3x97Vxe2x80x83xe2x80x83(4)
Yr2=(xcex8xc3x97V)/(Nxc3x97L)xe2x88x92Kh2xc3x97Gyxc3x97Vxe2x80x83xe2x80x83(5)
Therefore, in order to suppress an increase in the yaw rate Yr, expressed by Yr2 greater than Yr1 in the aforementioned phenomenon, the value of the amount of change in the yaw rate expressed by Equation 6 below need only be made smaller.
Yr2xe2x88x92Yr1=(Kh1xe2x88x92Kh2)xc3x97Gyxc3x97Vxe2x80x83xe2x80x83(6)
There is a tendency for the stability factor Kh to become larger as more braking force is distributed to the front wheels. It is thus understood that in order to suppress an increase in the yaw rate Yr after braking at turning of the vehicle, the value (Kh1xe2x88x92Kh2) needs only be made smaller as the product of Gy (lateral acceleration) and V (vehicle speed) becomes larger.
In the brake control system according to the aspect of the invention, the brake control system controls the braking force of the front wheels or rear wheels such that more braking force is distributed to the front wheels than to the rear wheels, as the product of the lateral acceleration of the vehicle and the vehicle speed becomes greater. As a result, an increase in the yaw rate when braking the vehicle while turning can be easily and effectively prevented without requiring complicated calculations. Further, because more braking force is distributed to the front wheels than to the rear wheels due to the fact that the braking force of at least the front wheels is increased, it is possible to reliably prevent a reduction in deceleration of the vehicle caused by a reduction in braking force of the entire vehicle.
In the brake control system according to the aspect of the invention, when the product of the lateral acceleration of the vehicle and the vehicle speed is equal to, or greater than, a first reference value and less than a second reference value, the braking force of the front wheels is increased. Further, when the product of the lateral acceleration of the vehicle and the vehicle speed is equal to, or greater than, the second reference value, the braking force of the front wheels is increased and the braking force of the rear wheels is decreased. Therefore, the possibility of the braking force of the front wheels becoming excessive is reduced while more braking force is reliably distributed to the front wheels than to the rear wheels.
In the brake control system according to the aspect of the invention, when the amount of braking operation applied by the vehicle operator is large, the increase amount of the braking force applied to the front wheels, which is based on the product of the lateral acceleration of the vehicle and the vehicle speed, is corrected such that the braking force of the front wheels is prevented from exceeding a predetermined range. Therefore, it is possible to reliably reduce the possibility of a decrease in the turning characteristics of the vehicle due to the braking force of the front wheels becoming excessive such that the lateral force of the front wheels is reduced when a large amount of braking operation is applied by the vehicle operator.
In the brake control system according to the aspect of the invention, it is determined whether the vehicle is in a state of turning while braking. If the vehicle is in a state of turning while braking, the braking force of the front wheels or rear wheels is controlled based on the product of the lateral acceleration of the vehicle and the vehicle speed. Therefore, it is possible to both reliably prevent the yaw rate of the vehicle from increasing when the vehicle is in a state of turning while braking, as well as reliably prevent distribution control of the braking force from being performed unnecessarily when the vehicle is not in a state of turning while braking.
According to an exemplary embodiment of the aspect of the invention, the controller calculates a target braking force applied to each of the front wheels and the rear wheels on the basis of a braking amount of the vehicle by an operator of the vehicle, and corrects the target braking force applied to the front wheels so as to be increased as an absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle increases.
According to another exemplary embodiment of the aspect of the invention, a rate of increase in the braking force applied to the front wheels, when the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than the second predetermined value, is smaller than the rate of increase in the braking force applied to the front wheels when the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than the first predetermined value and less than the second predetermined value.
According to another exemplary embodiment of the aspect of the invention, when the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than the second predetermined value, the controller reduces the braking force applied to one of the rear wheels on the inside of turning of the vehicle as the absolute value of the product increases.
According to another exemplary embodiment of the aspect of the invention, when the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle is equal to or greater than the second predetermined value, the controller reduces the braking force applied to one of the rear wheels on the inside of turning of the vehicle as the absolute value of the product becomes larger, and increases the braking force applied to the other of the rear wheels on the outside of turning of the vehicle as the absolute value of the product.
According to another exemplary embodiment of the aspect of the invention, the controller determines whether a road on which the vehicle is travelling has an uneven friction coefficient that is different in right and left areas of the road which correspond to the respective right and left wheels, and inhibits controlling of the braking force applied to the at least one of the front wheels and the rear wheels on the basis of the absolute value of the product of the detected lateral acceleration and the detected speed of the vehicle when it is determined that the road has the uneven friction coefficient that is different in right and left areas of the road.